1. Technical Field
The present invention relates generally to ultrasound visualization techniques and more particularly to catheter or endoscope based ultrasound microscopes.
2. Prior Art
Conventional catheters have been designed with ultrasound transducers operating at between 5 and 30 MHz which are introduced inside the bodies of patients. These low frequencies of ultrasound can create images to depths of several centimeters inside tissues. For a given transducer diameter, the axial resolution of an ultrasound beam increases as the frequency increases. The beam resolution along its axis is increased when pulse duration is decreased, by transmitting a pulse with increased bandwidth. Transducer diameter, frequency, bandwidth and sensitivity must be optimized simultaneously when designing ultrasound transducers.
Conventionally, 2.25 or 3.0 MHz transducers are used to image cardiac structures such as the mitral, tricuspid and aortic valves and left ventricular walls. Transducers operating at 5 MHz placed on infants and childrens bodies provide better resolution, although the beam penetration depth is less than with lower frequencies. Transducers operating at 5 or 7.5 MHz can image superficial structures such as the carotid and femoral arteries. Transesophageal echocardiography utilizes 5 or 7.5 MHz transducers with considerably improved image quality due to the higher frequencies and shorter distances from the transducer to the heart. Higher 20 to 30 MHz frequency transducers have been mounted on the tips of cardiac catheters for intravascular imaging, and have provided excellent resolution of the intima, media and adventitia of arterial walls. Transducers operating at 2 to 30 MHz frequencies provide excellent images of the heart and other organs, but still cannot image cellular detail.
When transducer frequency is increased to 1000 MHz, the ultrasound wavelength is significantly decreased and approaches that of light. The image resolution achievable with ultrasound at 1000 MHz is approximately 1 to 1.5 microns, which enables imaging cellular detail. Transducers operating at 600 to 1000 MHz can accurately assess myocardial cellular detail.
Prior art has imaged cellular characteristics in thin (5 micron) sections of tissue. Myocardial pathology, which is characterized by myocyte necrosis, lymphocytic infiltration and interstitial fibrosis, can be visualized clearly with very high frequency ultrasound. However, ultrasound in the very high frequency range trades off the disadvantage of very shallow penetration into tissue.
U.S. Pat. No. 4,546,771 by Eggleton et al. discloses a biopsy needle for positioning an ultrasound transducer capable of producing and receiving high (100 to 500 MHz) frequency ultrasound. Passing the needle into tissues enables microscopic examination of tissues within living bodies. Inside the needle a transducer generates an ultrasonic beam which is directed axially, then reflected off a mirror located at 45.degree. in the needle to be directed radially from the needle and focused at a point in the tissue. The tissue backscatters ultrasound toward the needle, where the 45.degree. mirror reflects it up along the needle axis to be received by the transducer. A piezoelectric bimorph actuator can reciprocate the mirror along the needle axis to produce a sequence of parallel A-scans, for forming a B-scan in a plane parallel to the needle axis. A second embodiment utilizing a rotary actuator can oscillate the mirror around the needle axis. This produces a sequence of radial A-scans for forming a B-scan in a plane perpendicular to the needle axis. Eggleton emphasizes frequencies between 400 and 500 MHz.
In the inventors experience, 400 MHz or higher frequency transducers are not satisfactory for imaging cells in thicksample sections of tissue. This is due to the extremely shallow depth of penetration of ultrasound at such high frequencies. In-vivo ultrasonic imaging of details of cells from thick sections is not known to have been reported.